System and methodology for prosody modification

ABSTRACT

A prosody modification system and methodology calculates synchronization marks in an original, quasi-periodic signal to a finer precision than the sampling rate of the original signal. Synthetic synchronization marks are generated according to the desired prosody modification also to a finer precision than the sampling rate of the original signal. Waveforms are extracted from the original signal and are fine-shifted to the exact location on the synthetic time axis by a resampling technique. The fine-shifted waveforms are windowed by an asymmetric filtering window, overlapped, and summed together to produce a synthetic signal.

RELATED APPLICATIONS

This. application claims the benefit of U.S. Provisional Application No.60/036,228, entitled “Method and System of Modifying Pitch Contour ofSpeech,” filed on Jan. 27, 1997 by Francisco M. Gimenez de los Galanes,incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to signal processing and, moreparticularly, to prosody modification of a quasi-periodic signal.

BACKGROUND OF THE INVENTION

Prosody modification is the adjustment of a quasi-periodic signalwithout affecting the timbre. Quasi-periodic signals include humanspeech, e.g., talking and singing, synthetic speech, and sounds frommusical instruments, such as notes from woodwind, brass, or stringedinstruments. Specific examples of prosody modification include adjustingthe pitch of a quasi-periodic signal without affecting the timbre, forexample, changing a sampled clarinet note from a C to a B while stillsounding like a clarinet. Another purpose of prosody modification is tochange the duration of a quasi-periodic signal without affecting eitherthe pitch or the timbre.

Practical applications of prosody modification include adding emphasisto portions of a pre-recorded message and changing the duration of humandialog to fit a particular time slot, e.g., an advertising announcementor lip-syncing during postproduction of a movie or video. Prosodymodification is also used to adjust the pitch of a singer or musicalinstrument, for example, to change the musical key, add vibrato, orcorrect for poor voice control. Speech synthesis requires prosodymodification of short speech segments before concatenation to createwords and longer messages.

One conventional approach to prosody modification is a pitch-synchronousoverlap-and-add technique. U.S. Pat. No. 5,524,172 describes aconventional overlap-and-add system for modifying the prosody of speechsynthesis segments, which are derived from human sounds sampled at arelatively low sampling rate of 16 kHz due to tight constraints incomputation and storage costs. A series of original synchronizationmarks within the speech segment are indexed by sample number and savedin a memory. The duration of the speech segments is modified bytime-warping the synchronization marks to produce a series of syntheticsynchronization marks, also indexed by a sample number. Waveforms areextracted from the speech segment at the original synchronization markusing a symmetrical Hanning window, overlapped by shifting to thecorresponding synthetic synchronization mark, and added to the outputsignal.

Conventional overlap-and-add techniques introduce some noise in the formof artificial jitter or harmonic mix-up, into the signal, which is heardas a “fuzziness” or a reedy quality. In particular, higher pitchedsignals, such as women's voices, children's voice, singing voices, andmost musical instrument notes, are especially affected. Moreover,conventional overlap-and-add systems have difficulty with signalsinvolving rapid changes in pitch, for example, during music such assigning or playing musical instruments.

SUMMARY OF THE INVENTION

There exists a need for a prosody modification system and methodologythat reduces the introduction of noise or fuzziness in its outputs.There is also a need for effectively modifying the prosody of signalswithout severely affecting the musicality or compromising the desiredpitch, for example, in higher-pitched signals, such as women's voices,children's voice, singing voices, and most musical instrument notes andsignals involving rapid changes in pitch.

One aspect of the present invention stems from the realization that animportant source of errors in the output signal of conventionaloverlap-and-add systems is due to the rounding synchronization of thewaveforms to intervals defined by the relatively low sampling rate.However, it is not desirable to increase the sampling rate owing to thetight computational and storage constraints.

Accordingly, one aspect of the present invention is a method andcomputer-readable medium bearing instructions for performing a prosodymodification on a quasi-periodic signal, sampled at a sampling interval.A series of original synchronization marks is determined for thequasi-periodic signal, from which a series of synthetic synchronizationmarks are determined in accordance with the prosodic modification.Waveforms are extracted from the quasi-periodic signal around one of theoriginal synchronization marks, and shifted to one of the syntheticsynchronization marks corresponding to the original synchronizationmarks. The difference of the original synchronization mark and thesynthetic synchronization mark is not an integral multiple of saidsampling interval. One implementation of non-integral shifting is byresampling the quasi-periodic signal. The prosody-modified signal isthen generated based on the shifted waveforms, for example, byoverlap-and-add techniques.

Another aspect of the present invention stems from the realization thatanother source of errors in conventional overlap-and-add techniques isthe use of symmetric windows in extracting waveforms aroundsynchronization marks when the pitch is rapidly changing. The symmetricwindows tend to either extract too little or too much of the waveform tobe overlapped-and-added.

Accordingly, a method and computer-readable medium bearing instructionsare provided for synthesizing a quasi-periodic signal from an originalsignal. A series of original synchronization marks is determined for thequasi-periodic signal, from which a series of synthetic synchronizationmarks are determined in accordance with the prosodic modification.Waveforms are extracted from around one of-the original synchronizationmarks by applying an asymmetric filtering window and time-shifting thewaveforms according to the original synchronization mark and acorresponding synthetic synchronization marks. The extracted, shiftedwaveforms are summed to synthesize the quasi-periodic signal. Thefiltering window may be defined as having a first half-width on one sideof the original synchronization mark and a second half-width on anotherside of the original synchronization mark, in which the first half-widthis different from the second half-width. In some implementations, thefiltering window comprises two half-Hanning windows.

Additional needs, objects, advantages, and novel features of the presentinvention will be set forth in part in the description that follows, andin part, will become apparent upon examination or may be learned bypractice of the invention. The objects and advantages of the inventionmay be realized and obtained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 schematically depicts a computer system that can implement thepresent invention;

FIG. 2 is a flowchart illustrating the operation of an embodiment of thepresent invention; and

FIGS. 3(a) and 3(b) depict an exemplary sampled signal with an originalsynchronization mark and a synthetic synchronization mark.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A method and apparatus for prosody modification is described. In thefollowing description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-knownstructures and devices are shown in block diagram form in order to avoidunnecessarily obscuring the present invention.

HARDWARE OVERVIEW

FIG. 1 is a block diagram that illustrates a computer system 100 uponwhich an embodiment of the invention may be implemented. Computer system100 includes a bus 102 or other communication mechanism forcommunicating information, and a processor (or a plurality of centralprocessing units working in cooperation) 104 coupled with bus 102 forprocessing information. Computer system 100 also includes a main memory106, such as a random access memory (RAM) or other dynamic storagedevice, coupled to bus 102 for storing information and instructions tobe executed by processor 104. Main memory 106 also may be used forstoring temporary variables or other intermediate information duringexecution of instructions to be executed by processor 104. Computersystem 100 further includes a read only memory (ROM) 108 or other staticstorage device coupled to bus 102 for storing static information andinstructions for processor 104. A storage device 110, such as a magneticdisk or optical disk, is provided and coupled to bus 102 for storinginformation and instructions.

Computer system 100 may be coupled via bus 102 to a display 111, such asa cathode ray tube (CRT), for displaying information to a computer user.An input device 113, including alphanumeric and other keys, is coupledto bus 102 for communicating information and command selections toprocessor 104. Another type of user input device is cursor control 115,such as a mouse, a trackball, or cursor direction keys for communicatingdirection information and command selections to processor 104 and forcontrolling cursor movement on display 111. This input device typicallyhas two degrees of freedom in two axes, a first axis (e.g., x) and asecond axis (e.g., y), that allows the device to specify positions in aplane. For audio output and input, computer system 100 may be coupled toa speaker 117 and a microphone 119, respectively.

The invention is related to the use of computer system 100 for prosodymodification. According to one embodiment of the invention, prosodymodification is provided by computer system 100 in response to processor104 executing one or more sequences of one or more instructionscontained in main memory 106. Such instructions may be read into mainmemory 106 from another computer-readable medium, such as storage device110. Execution of the sequences of instructions contained in main memory106 causes processor 104 to perform the process steps described herein.One or more processors in a multi-processing arrangement may also beemployed to execute the sequences of instructions contained in mainmemory 106. In alternative embodiments, hard-wired circuitry may be usedin place of or in combination with software instructions to implementthe invention. Thus, embodiments of the invention are not limited to anyspecific combination of hardware circuitry and software.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to processor 104 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media, volatile media, and transmission mediaNon-volatile media include, for example, optical or magnetic disks, suchas storage device 110. Volatile media include dynamic memory, such asmain memory 106. Transmission media include coaxial cables, copper wireand fiber optics, including the wires that comprise bus 102.Transmission media can also take the form of acoustic or light waves,such as those generated during radio frequency (RF) and infrared (IR)data communications. Common forms of computer-readable media include,for example, a floppy disk, a flexible disk, hard disk, magnetic tape,any other magnetic medium, a CD-ROM, DVD, any other optical medium,punch cards, paper tape, any other physical medium with patterns ofholes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip orcartridge, a carrier wave as described hereinafter, or any other mediumfrom which a computer can read.

Various forms of computer readable media may be involved in carrying oneor more sequences of one or more instructions to processor 104 forexecution. For example, the instructions may initially be borne on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 100 canreceive the data on the telephone line and use an infrared transmitterto convert the data to an infrared signal. An infrared detector coupledto bus 102 can receive the data carried in the infrared signal and placethe data on bus 102. Bus 102 carries the data to main memory 106, fromwhich processor 104 retrieves and executes the instructions. Theinstructions received by main memory 106 may optionally be stored onstorage device 110 either before or after execution by processor 104.

Computer system 100 also includes a communication interface 120 coupledto bus 102. Communication interface 120 provides a two-way datacommunication coupling to a network link 121 that is connected to alocal network 122. Examples of communication interface 120 include anintegrated services digital network (ISDN) card, a modem to provide adata communication connection to a corresponding type of telephone line,and a local area network (LAN) card to provide a data communicationconnection to a compatible LAN. Wireless links may also be implemented.In any such implementation, communication interface 120 sends andreceives electrical, electromagnetic or optical signals that carrydigital data streams representing various types of information.

Network link 121 typically provides data communication through one ormore networks to other data devices. For example, network link 121 mayprovide a connection through local network 122 to a host computer 124 orto data equipment operated by an Internet Service Provider (ISP) 126.ISP 126 in turn provides data communication services through the worldwide packet data communication network, now commonly referred to as the“Internet” 128. Local network 122 and Internet 128 both use electrical,electromagnetic or optical signals that carry digital data streams. Thesignals through the various networks and the signals on network link 121and through communication interface 120, which carry the digital data toand from computer system 100, are exemplary forms of carrier wavestransporting the information.

Computer system 100 can send messages and receive data, includingprogram code, through the network(s), network link 121 and communicationinterface 120. In the Internet example, a server 130 might transmit arequested code for an application program through Internet 128, ISP 126,local network 122 and communication interface 118. In accordance withthe invention, one such downloaded application provides for prosodymodification as described herein. The received code may be executed byprocessor 104 as it is received, and/or stored in storage device 110, orother non-volatile storage for later execution. In this manner, computersystem 100 may obtain application code in the form of a carrier wave.

PROSODY MODIFICATION

FIG. 2 is a flowchart illustrating the operation of prosody modificationof an original quasi-periodic signal into a synthetic signal, accordingto one embodiment of the present invention. In step 200, a series oforiginal synchronization marks is established for the original signal.In contrast to conventional methodologies, the original synchronizationmarks are calculated to a greater precision than the sampling rate underwhich the original signal is processed. For example, if the processingsampling rate is 16 kHz, synchronization marks in the original signalmay be established to a resolution of 21 μs, although the signal issampled for processing in intervals of about 63 μs. One approach to isto determine the synchronization mark on an upsampled version of theoriginal signal, for example, at a rate that is at least three timesfaster than the processing sampling rate. Another approach, which doesnot use upsampling but mathematical curve fitting, is described in moredetail herein below.

Referring to FIG. 3(a), a sampled, quasi-periodic signal is depicted, inwhich an original synchronization mark 310 is located between sample 300and sample 302. Sample 300 is an amplitude of the original,quasi-periodic signal at an instant in time, and sample 302 is anamplitude of the same quasi-periodic signal at a later instant in time.The interval between sample 300 and sample 302 is the sampling period.Original synchronization mark 310 is calculated to a finer resolutionthan the sampling rate, and therefore is not necessarily coincident withany of the samples in the sampled original signal. In FIG. 3(a),original synchronization mark 310 is roughly 80% of the way from sample300 to sample 302.

The original synchronization marks can be established by a variety ofmeans, and, for human speech, the synchronization marks are preferablyaligned to glottal closure instants, called “epochs.” An epoch occurswhen the glottis, which is the space between the vocal cords at theupper part of the larynx, closes and causes a “ring-down” damping effectin the vocal signal. A convenient definition of the time of glottalclosure is the instant at which there is a maximum rate of change in theairflow through the glottis. One approach to finding the epochs is byapplication of standard epoch detection methods on an upsampled versionof the original signal, for example, at about 48 kHz. Another approachto finding the epochs, also on an upsampled signal, uses fundamentalfrequency tracking as described in D. Talkin, “A Robust Algorithm forPitch Tracking (RAPT), “Speech Coding & Synthesis, Kleijn & Paliwaleds., (Amsterdam: Elsevier, 1995), in which a fundamental frequency f₀is detected using cross-correlation and dynamic programming techniques.The detected fundamental frequency is combined with peaks picked from anintegrated linear predictive coding residual in a dynamic programmingframework that finds the set of epochs most consistent with the localestimates of the fundamental frequency f₀. Still another approach, whichdoes not involve explicit upsampling, is to fit a function such as apolynomial to the speech signal in the vicinity of the peak, and thenuse analytic techniques to find the peak in the function nearest thecoarse epoch estimate obtained at the original sampling rate.

Referring back to FIG. 2, in step 202, a series of syntheticsynchronization marks is generated based on prosody modificationinformation such as a desired fundamental frequency contour and adesired time-warping function, as by iteratively integrating the desiredfundamental frequency contour and the desired time-warping function. Thetime-warping function establishes a projection of the original andsynthetic time axes that determines a frame-level mapping from segmentsof the original waveform to a time on the synthetic axis. When thecombination of the fundamental frequency and the time-scale modificationimplies a denser or sparser set of synchronization marks, frames arerepeated or omitted, respectively, to compensate.

Unlike conventional techniques, the synthetic synchronization marks arenot quantized to the signal sampling frequency intervals, but to a finerresolution than the sampling interval, preferably limited only by theprecision of the underlying hardware. For example, the mantissa of a32-bit floating number provides 24 bits of resolution. Referring to FIG.3(b), a synthetic synchronization mark 320 is depicted lying betweensample 300 and sample 302. The synthetic synchronization mark 320 willnot generally occur at the same location of the corresponding originalsynchronization mark 310 and will be offset from the originalsynchronization mark 310 by some delay δ. Delay δ is not necessarily anintegral multiple of the sampling interval (the period between sample300 and sample 302), and in fact may be a fraction of one samplinginterval.

GENERATING SYNTHETIC FRAMES

After the original and synthetic synchronization marks are generated,waveforms from the original signal are extracted by applying a filteringwindow around an original synchronization mark in step 204. Thisfiltering window can be a rectangular window that defines a frame fromthe previous synchronization mark to the next synchronization mark.Thus, a frame comprises two periods: the first period from the previoussynchronization mark to the current synchronization mark, and the secondperiod from the current synchronization mark to the next synchronizationmark. However, other implementations may employ a raised cosine windowsuch as a Hamming window, a symmetric Hanning window, or an asymmetricHanning window, which is described in more detail herein below inconjunction with step 210, or other center-weighted window.

After waveforms in the selected frame are extracted from the originalsignal from around an original synchronization mark, the waveforms areshifted to the corresponding synthetic synchronization mark. Accordingto one embodiment of the present invention, the extracted waveforms areshifted by a two-step process. First, the selected frame is shifted tothe closest sampling interval that is before the syntheticsynchronization mark (step 206), as by conventional techniques.

The second step is a fine-shifting step that moves the frame to theexact position in time for the synthetic synchronization mark (step208). One approach to fine-shifting is to reconstruct the originalsignal from its samples and resample the original signal again afterintroducing the desired delay in the analog domain. The resampling ofthe original signal can be performed digitally by upsampling the digitalsignal (i.e., the sampled original signal), applying a digitalreconstruction filter at that higher sampling rate, introducing aninteger delay at that upsampling rate, and downsampling the delayedsignal down to the original sampling rate. The upsampling rate isdetermined by the admissible quantization of the delay at the highersampling rate. Using a sinc(x) reconstruction filter, the resampledsignal can be expressed by the following equation: $\begin{matrix}{{{y\lbrack m\rbrack} = {\sum\limits_{n = {- \infty}}^{\infty}{{x\lbrack n\rbrack}\left( \frac{\sin \quad \alpha \quad \pi}{\pi} \right)\frac{- 1^{({m - n})}}{\left( {m - n} \right) + \alpha}}}},} & (1)\end{matrix}$

where x[n] is the gross-shifted original signal, y[m] is thefine-shifted signal, and α is the quotient of the fine delay δ and thesampling period T_(s). In practice, the limits of the summation areconstrained to a sensible integer value such as 40, which introducessome distortion in the resulting signal. This distortion, however, canbe reduced by applying a tapering window as explained in F. M. Gimenezde los Galanes et al., “Speech Synthesis System Based on a VariableDecimation/Interpolation Factor,” IEEE Proc. ICASSP '95 (Detroit: 1995).Other prosody modifications may be applied at this point, for example,controlling emphasis by multiplying the waveforms by a gain factor.

SIGNAL SYNTHESIS

After the extracted waveforms have been fine-shifted, the shiftedwaveforms are combined to produce the synthesized signal, preferably byapplication of the following, overlap-and-add technique to account forrapid changes in pitch. In step 210, an asymmetric window is applied toextract an overlapping frame. More specifically, according to oneembodiment of the present invention, the first section of the asymmetricwindow is half of a Hanning window, increasing in amplitude from 0 to anon-zero value such as 1, with a length that is the lesser of the lengthof the first original period and the first synthetic period. The secondsection of the asymmetric window is half of a Hanning window, decreasingin amplitude from the non-zero value to 0, with a length that is thelesser of the length of the second original period and the secondsynthetic period. It is evident that other filtering windows may beemployed, for example, an inherently asymmetric window such as a gammafunction or halves of symmetric windows such as a Hamming window orother raised cosine window. The asymmetric windowing strategy reducesthe distortion in the windowing step of an overlap-and-add technique bynot extracting too little or too much of the waveform.

In the embodiment of the present invention illustrated in the flowchartof FIG. 2, the asymmetric windowing is applied to a time-shiftedwaveform. However, in another embodiment of the present invention, thewaveform is first extracted by an asymmetric window and thentime-shifted, even by conventional techniques. After the windowed,time-shifted waveform is extracted, it is summed with other overlappingwindowed, time-shifted waveforms to create the synthetic signal inaccordance with conventional overlap-and-add techniques (step 212).

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A method of performing a prosody modification ona quasi-periodic signal, sampled at a sampling interval, to produce amodified signal, said method comprising the machine-implemented stepsof: determining a series of original synchronization marks in saidquasi-periodic signal; determining a series of synthetic synchronizationmarks based on said original synchronization marks and said prosodicmodification; extracting waveforms from said quasi-periodic signalaround one of said original synchronization marks; shifting saidwaveforms to one of said synthetic synchronization marks correspondingto said one of said original synchronization marks to produce shiftedwaveforms, wherein a difference of said one of said originalsynchronization marks and said one of said synthetic synchronizationmarks is a non-integral multiple of said sampling interval; andgenerating said modified signal based on said shifted waveforms.
 2. Amethod as in claim 1, wherein the step of determining a series oforiginal synchronization marks in said quasi-periodic signal includesthe step of determining at least one of said original synchronizationmarks at a resolution finer than the sampling interval.
 3. A method asin claim 2, wherein the step of determining at least one of saidoriginal synchronization marks at a resolution finer than the samplinginterval includes the step of sampling the quasi-periodic signal at ashorter sampling interval with respect to said sampling interval.
 4. Amethod as in claim 2, wherein the step of determining at least one ofsaid original synchronization marks at a resolution finer than thesampling interval includes fitting a mathematical curve to find a peakin said quasi-periodic signal.
 5. A method as in claim 3, wherein saidshorter sampling interval is at most one-third of said samplinginterval.
 6. A method as in claim 1, wherein the step of determining aseries of original synchronization marks in said quasi-periodic signalincludes the step of determining epochs in said quasi-periodic signal.7. A method as in claim 1, wherein the step of determining a series ofsynthetic synchronization marks includes the step of determining atleast one of said synthetic synchronization marks at a resolution finerthan the sampling interval.
 8. A method as in claim 7, wherein the stepof determining at least one of said synthetic synchronization marks at aresolution finer than the sampling interval includes the step ofdetermining said at least one of said synthetic synchronization marks bya floating point number having a mantissa of at least twenty-four bits.9. A method as in claim 1, wherein the step of shifting said waveformsto one of said synthetic synchronization marks corresponding to said oneof said original synchronization marks includes the step of resamplingsaid waveforms to adjust said waveforms to said one of said syntheticsynchronization marks.
 10. A method as in claim 9, wherein the step ofshifting said waveforms to one of said synthetic synchronization markscorresponding to said one of said original synchronization marks furtherincludes the step of shifting said waveforms to the nearest previoussampling interval of said one of said synthetic synchronization marks,before said step of resampling is performed.
 11. A method as in claim 1,wherein the step of generating said modified signal based on saidshifted waveforms includes the steps of: applying an asymmetricfiltering window to said shifted waveforms; and summing the windowed,shifted waveform to generate said modified signal.
 12. A method as inclaim 11, wherein: said asymmetric filtering window has a first sectionand a second section in juxtaposition with each other; said firstsection has an amplitude progressively increasing from zero to anon-zero value along a first width; said second section has an amplitudeprogressively decreasing from said non-zero value to zero along a secondwidth; and said first width is different in size from said second width.13. A method as in claim 12, wherein: said first width is the lesser ofthe interval between said one of said original synchronization marks anda preceding original synchronization mark and the interval between saidone of said synthetic synchronization marks and a preceding syntheticsynchronization mark; and said second width is the lesser of theinterval between said one of said original synchronization marks and asubsequent original synchronization mark and the interval between saidone of said synthetic synchronization marks and a subsequent syntheticsynchronization mark.
 14. A method as in claim 13, wherein: said firstsection is the first half of a Hanning window; and said second sectionis the second half of a Hanning window.
 15. A method of synthesizing aquasi-periodic signal from an original signal, said method comprisingthe steps of: determining a series of original synchronization marks insaid original signal; determining a series of synthetic synchronizationmarks based on said original synchronization marks and on prosodyinformation; extracting a waveform from around each of said originalsynchronization marks by applying a filtering window and time-shiftingeach waveform according to a respective one of said originalsynchronization marks and a respective one of said syntheticsynchronization marks corresponding to said respective one of saidoriginal synchronization marks, wherein each filtering window has afirst half-width on one side of a respective original synchronizationmark and a second half-width on another side of the respective originalsynchronization mark, and said first half-width is the lesser of theinterval between said respective one of said original synchronizationmarks and a preceding original synchronization mark and the intervalbetween said respective one of said synthetic synchronization marks anda preceding synthetic synchronization mark; and summing the extractedwaveforms to synthesize said quasi-periodic signal.
 16. A method as inclaim 15, wherein said step of windowing is performed before said stepof time-shifting.
 17. A method as in claim 15, wherein: said filteringwindow has a first section and a second section in juxtaposition witheach other; said first section has an amplitude progressively increasingfrom zero to a non-zero value along said first half-width; and saidsecond section has amplitude progressively decreasing from said non-zerovalue to zero along said second half-width.
 18. A method as in claim 17,wherein: said second half-width is the lesser of the interval betweensaid one of said original synchronization marks and a subsequentoriginal synchronization mark and the interval between said one of saidsynthetic synchronization marks and a subsequent syntheticsynchronization mark.
 19. A method as in claim 18, wherein: said firstsection is the first half of a Hanning window; and said second sectionis the second half of a Hanning window.
 20. A method as in claim 15,wherein said step of windowing is performed after said step oftime-shifting.
 21. A method as in claim 15, wherein a difference of saidone of said original synchronization marks and said one of saidsynthetic synchronization marks is a non-integral multiple of saidsampling interval.
 22. A method as in claim 21, wherein the step ofdetermining a series of original synchronization marks in saidquasi-periodic signal includes the step of determining at least one ofsaid original synchronization marks at a resolution finer than thesampling interval.
 23. A method as in claim 22, wherein the step ofdetermining at least one of said original synchronization marks at aresolution finer than the sampling interval includes the step ofsampling the quasi-periodic signal at a shorter sampling interval withrespect to said sampling interval.
 24. A method as in claim 23, whereinsaid shorter sampling interval is at most one-third of said samplinginterval.
 25. A method as in claim 21, wherein the step of determining aseries of original synchronization marks in said quasi-periodic signalincludes the step of determining epochs in said quasi-periodic signal.26. A method as in claim 21, wherein the step of determining a series ofsynthetic synchronization marks includes the step of determining atleast one of said synthetic synchronization marks at a resolution finerthan the sampling interval.
 27. A method as in claim 26, wherein thestep of determining at least one of said synthetic synchronization marksat a resolution finer than the sampling interval includes the step ofdetermining said at least one of said synthetic synchronization marks bya floating point number having a mantissa of at least twenty-four bits.28. A method as in claim 21, wherein the step of shifting said waveformsto one of said synthetic synchronization marks corresponding to said oneof said original synchronization marks includes the step of resamplingsaid waveforms to adjust said waveforms to said one of said syntheticsynchronization marks.
 29. A method as in claim 28, wherein step ofshifting said waveforms to one of said synthetic synchronization markscorresponding to said one of said original synchronization marks furtherincludes the step of shifting said waveforms to the nearest previoussampling interval of said one of said synthetic synchronization marks,before said step of resampling is performed.
 30. A computer-readablemedium bearing instructions for performing a prosody modification on aquasi-periodic signal, sampled at a sampling interval, to produce amodified signal, said instructions arranged, when executed, to cause oneor more processors to perform the steps of: determining a series oforiginal synchronization marks in said quasi-periodic signal;determining a series of synthetic synchronization marks based on saidoriginal synchronization marks and said prosodic modification;extracting waveforms from said quasi-periodic signal around one of saidoriginal synchronization marks; shifting said waveforms to one of saidsynthetic synchronization marks corresponding to said one of saidoriginal synchronization marks, wherein a difference of said one of saidoriginal synchronization marks and said one of said syntheticsynchronization marks is a non-integral multiple of said samplinginterval; and generating said modified signal based on said shiftedwaveforms.
 31. A computer-readable medium as in claim 30, wherein thestep of determining a series of original synchronization marks in saidquasi-periodic signal includes the step of determining at least one ofsaid original synchronization marks at a resolution finer than thesampling interval.
 32. A computer-readable medium as in claim 31,wherein the step of determining at least one of said originalsynchronization marks at a resolution finer than the sampling intervalincludes the step of sampling the quasi-periodic signal at a shortersampling interval with respect to said sampling interval.
 33. Acomputer-readable medium as in claim 32, wherein said shorter samplinginterval is at most one-third of said sampling interval.
 34. A method asin claim 31, wherein the step of determining at least one of saidoriginal synchronization marks at a resolution finer than the samplinginterval includes fitting a mathematical curve to find a peak in saidquasi-periodic signal.
 35. A computer-readable medium as in claim 30,wherein the step of determining a series of original synchronizationmarks in said quasi-periodic signal includes the step of determiningepochs in said quasi-periodic signal.
 36. A computer-readable medium asin claim 30, wherein the step of determining a series of syntheticsynchronization marks includes the step of determining at least one ofsaid synthetic synchronization marks at a resolution finer than thesampling interval.
 37. A computer-readable medium as in claim 36,wherein the step of determining at least one of said syntheticsynchronization marks at a resolution finer than the sampling intervalincludes the step of determining said at least one of said syntheticsynchronization marks by a floating point number having a mantissa of atleast twenty-four bits.
 38. A computer-readable medium as in claim 30,wherein the step of shifting said waveforms to one of said syntheticsynchronization marks corresponding to said one of said originalsynchronization marks includes the step of resampling said waveforms toadjust said waveforms to said one of said synthetic synchronizationmarks.
 39. A computer-readable medium as in claim 38, wherein the stepof shifting said waveforms to one of said synthetic synchronizationmarks corresponding to said one of said original synchronization marksfurther includes the step of shifting said waveforms to the nearestprevious sampling interval of said one of said synthetic synchronizationmarks, before performed said step of resampling.
 40. A computer-readablemedium as in claim 30, wherein the step of generating said modifiedsignal based on said shifted waveforms includes the steps of: applyingan asymmetric filtering window to said shifted waveforms; and summingthe windowed, shifted waveform to generate said modified signal.
 41. Acomputer-readable medium as in claim 40, wherein: said asymmetricfiltering window has a first section and a second section injuxtaposition with each other; said first section has an amplitudeprogressively increasing from zero to a non-zero value along a firstwidth; said second section has amplitude progressively decreasing fromsaid non-zero value to zero along a second width; and said first widthis different is size from said second width.
 42. A computer-readablemedium as in claim 41, wherein: said first width is the lesser of theinterval between said one of said original synchronization marks and apreceding original synchronization mark and the interval between saidone of said synthetic synchronization marks and a preceding syntheticsynchronization mark; and said second width is the lesser of theinterval between said one of said original synchronization marks and asubsequent original synchronization mark and the interval between saidone of said synthetic synchronization marks and a subsequent syntheticsynchronization mark.
 43. A computer-readable medium as in claim 42,wherein: said first section is the first half of a Hanning window; andsaid second section is the second half of a Hanning window.
 44. Acomputer-readable medium bearing instructions for synthesizing aquasi-periodic signal from an original signal, said instructionsarranged, when executed, to cause one or more processors to perform thesteps of: determining a series of original synchronization marks in saidoriginal signal; determining a series of synthetic synchronization marksbased on said original synchronization marks and on prosody information;extracting a waveform from around each of said original synchronizationmarks by applying a filtering window and time-shifting each waveformaccording to a respective one of said original synchronization marks anda respective one of said synthetic synchronization marks correspondingto said respective one of said original synchronization marks to form atime-shifted signal; applying asymmetric filtering windows to thetime-shifted signal to extract overlapping frames; and summing theoverlapping frames to synthesize said quasi-periodic signal.
 45. Acomputer-readable medium as in claim 44, wherein each said asymmetricfiltering window has a first half-width on one side of a respectiveoriginal synchronization mark and a second half-width on another side ofthe respective original synchronization mark, said first half-widthdifferent in size from said second half-width.
 46. A computer-readablemedium as in claim 45, wherein: said asymmetric filtering window has afirst section and a second section in juxtaposition with each other;said first section has an amplitude progressively increasing from zeroto a non-zero value along said first half-width; and said second sectionhas an amplitude progressively decreasing from said non-zero value tozero along said second half-width.
 47. A computer-readable medium as inclaim 46, wherein: said first half-width is the lesser of the intervalbetween said one of said original synchronization marks and a precedingoriginal synchronization mark and the interval between said one of saidsynthetic synchronization marks and a preceding syntheticsynchronization mark; and said second half-width is the lesser of theinterval between said one of said original synchronization marks and asubsequent original synchronization mark and the interval between saidone of said synthetic synchronization marks and a subsequent syntheticsynchronization mark.
 48. A computer-readable medium as in claim 47,wherein: said first section is the first half of a Hanning window; andsaid second section is the second half of a Hanning window.
 49. Acomputer-readable medium as in claim 44, wherein the step of windowingis performed after the step of time-shifting.
 50. A computer-readablemedium as in claim 45, wherein a difference of said one of said originalsynchronization marks and said one of said synthetic synchronizationmarks is a non-integral multiple of said sampling interval.
 51. Acomputer-readable medium as in claim 50, wherein the step of determininga series of original synchronization marks in said quasi-periodic signalincludes the step of determining at least one of said originalsynchronization marks at a resolution finer than the sampling interval.52. A computer-readable medium as in claim 51, wherein the step ofdetermining at least one of said original synchronization marks a t aresolution finer than the sampling interval includes the step ofsampling the quasi-periodic signal at a shorter sampling interval withrespect to said sampling interval.
 53. A computer-readable medium as inclaim 52, wherein said shorter sampling interval is at most one-third ofsaid sampling interval.
 54. A computer-readable medium as in claim 50,wherein the step of determining a series of original synchronizationmarks in said quasi-periodic signal includes the step of determiningepochs in said quasi-periodic signal.
 55. A computer-readable medium asin claim 50, wherein the step of determining a series of syntheticsynchronization marks includes the step of determining at least one ofsaid synthetic synchronization marks at a resolution finer than thesampling interval.
 56. A computer-readable medium as in claim 55,wherein the step of determining at least one of said syntheticsynchronization marks at a resolution finer than the sampling intervalincludes the step of determining said at least one of said syntheticsynchronization marks by a floating point number having a mantissa of atleast twenty-four bits.
 57. A computer-readable medium as in claim 50,wherein the step of shifting said waveforms to one of said syntheticsynchronization marks corresponding to said one of said originalsynchronization marks includes the step of resampling said waveforms toadjust said waveforms to said one of said synthetic synchronizationmarks.
 58. A computer-readable medium as in claim 57, wherein step ofshifting said waveforms to one of said synthetic synchronization markscorresponding to said one of said original synchronization marks furtherincludes the step of shifting said waveforms to the nearest previoussampling interval of said one of said synthetic synchronization marks,before performed said step of resampling.